Reading the Dwell meter

[desoto1939]

Ok, I went to a car show today and was able to purchase a Dwell and tach meter for my 39 Desoto.

 

I have the engine RPM set at 700 the car likes to run at that Idle speed setting it at 550 to 625 seem to not be real smooth.

 

Set the meter up to read the Dwell.  The dwell is reading around 45  but the manual states it should be between 35-38.

 

So what does this mean to me since is is approx 7-10 higher onthe meter.  I know that the Dwell is reading the opening closing of the points.

 

The point official setting is stated to be at a gap of 20 but I gap around the 18 to for my original IGS 4102c dizzy.  So since I am gapped on 18 and not the 20 is this affecting that the RPM needs to be at the 700 instead of the 600.

 

So trying to understand the dwell tuneup process always willing to learn more about my car.

 

The car runs fine and has power and runs down the road very nicely around 50-52 MPH without any problem or hesitations and climbs hill just fine.  So basically I learned a long time ago if it is not broke then do not try to mess with anything.

 

Rich Hartung

desoto1939@aol.com

 

 

[Sniper]

Some dwell meters have multiple scales and/or ranges for a 4, 6 or 8 cylinder.  We'll assume you have it setup correctly and are reading the right scale.

 

Smaller gaps increase dwell time.  Dwell time is how long the points are closed.

 

So your smaller gap should show more dwell time, though I can't say if 45 is where it ought to be reading with .018" gap.  When I got my 51 the gap was also too small, the dwell too much, the timing way retarded and the idle speed was at 1600 rpm.  All of those interact with each other to some extent so you will have to go bath and forth double checking till everything is settled down to where it needs to be.

 

 

[desoto1939]

Yes I had the dwell set for 6 cyinder and the proper red and black picjup points connected as per the instruction manual for the dwell meter.

 

So are you stating that I need to set the gap at 20? i hate to fool with things that are running well Any help or advise is greatly appreciated.

 

Rich Hartung

desoto1939@aol.com

[kencombs]

Like Sniper said,  dwell is points closed degrees.  So, a smaller gap will equal longer dwell.  .018 will create longer dwell time than .020, how much more, I can't say.  But, it's easy to set and reset to see how it works on your car.  A worn shaft bushing can affect the dwell as can a bad condenser. Well the condenser does't actually change the physical dwell, but can cause the reading to be off.  odd and rare but it does happen.   IME, 700 seems way to fast for a good engine to idle. 

[keithb7]

When using a dwell meter, I generally overlook the actual measured gap. I don't bother attempting to learn what actual gap measures in at by using a feeler gauge. I use the dwell meter to measure the dwell time.  I adjust according to what the meter shows. Points surfaces become mis-aligned as they wear. A feeler gauge can produce inaccurate recordings when measuring worn points.  If you happen to have a Motors Manual there is good info in the general repair section under the Ignition System title.    One example is provided in the illustration below. The dwell meter is a more accurate precise measurement. A feeler gauge can be accurate on flat aligned points. When points are worn, the feeler gauge often only gets you in the ball park.

 

If your car seems to be running fine and you are hesitant to tweak the dwell, you don't have too. The dwell time is the amount of time the points are closed. My understanding is this is the amount of time that the coil can build up a high electrical discharge. Generally, more dwell time attributes to higher voltage at the pug tip when it arcs. A nice thick, hot blue spark. Especially important under a heavy torque load such as a hill climb. I deduce that too much dwell introduces the laws of diminishing returns.

 

Timing should be checked and re-set after the dwell is adjusted. Additionally any changes to the air idle mixture screw will also effect idle speed. A vacuum gauge is helpful here too to set the idle mixture. Then adjust the dwell and idle speed. Then the timing light comes out. Worn distributor shaft bushings will have the timing bouncing around. Seen when using a timing light. Additionally a sloppy worn breaker plate may also net erratic timing. The condenser, the tricky little variable that can also be troublesome, I just go ahead and change out if I do't recall or know the history of. I've had good luck with decent quality condensers that work. Others will tell all sorts of stories about cheap, crappy, brand new, condensers they had to deal with.

 

In addition to all those variable...Theres the coil, cap, rotor, spark-plugs and wires, vacuum and mechanical advance mechanisms.  Points spring arm tension. Lol, a can of worms! It is fun though. Sorry I'm getting off topic here.

 

Keith

 

Edited October 19, 2020 by keithb7

[Sniper]

16 hours ago, keithb7 said:

When using a dwell meter, I generally overlook the actual measured gap. I don't bother attempting to learn what actual gap measures in at by using a feeler gauge. I use the dwell meter to measure the dwell time.  I adjust according to what the meter shows. Points surfaces become mis-aligned as they wear. A feeler gauge can produce inaccurate recordings when measuring worn points.  If you happen to have a Motors Manual there is good info in the general repair section under the Ignition System title.    One example is provided in the illustration below. The dwell meter is a more accurate precise measurement. A feeler gauge can be accurate on flat aligned points. When points are worn, the feeler gauge often only gets you in the ball park.

 

If your car seems to be running fine and you are hesitant to tweak the dwell, you don't have too. The dwell time is the amount of time the points are closed. My understanding is this is the amount of time that the coil can build up a high electrical discharge. Generally, more dwell time attributes to higher voltage at the pug tip when it arcs. A nice thick, hot blue spark. Especially important under a heavy torque load such as a hill climb. I deduce that too much dwell introduces the laws of diminishing returns.

 

Timing should be checked and re-set after the dwell is adjusted. Additionally any changes to the air idle mixture screw will also effect idle speed. A vacuum gauge is helpful here too to set the idle mixture. Then adjust the dwell and idle speed. Then the timing light comes out. Worn distributor shaft bushings will have the timing bouncing around. Seen when using a timing light. Additionally a sloppy worn breaker plate may also net erratic timing. The condenser, the tricky little variable that can also be troublesome, I just go ahead and change out if I do't recall or know the history of. I've had good luck with decent quality condensers that work. Others will tell all sorts of stories about cheap, crappy, brand new, condensers they had to deal with.

 

In addition to all those variable...Theres the coil, cap, rotor, spark-plugs and wires, vacuum and mechanical advance mechanisms.  Points spring arm tension. Lol, a can of worms! It is fun though. Sorry I'm getting off topic here.

 

Keith

 

 

 

This is what points files were invented for.  When I worked as an I&E tech at a cement plant we'd had to disassemble and clean the contacts on the motor contactors about once a quarter.  Essentially, they are just large points and we did it for years with zero issues.  A light touch to dress them is all that is needed.

[Frank Elder]

17 hours ago, keithb7 said:

Points surfaces become mis-aligned as they wear. A feeler gauge can produce inaccurate recordings when measuring worn points.  

 

Keith

@desoto1939 This is very important information, as you are a very meticulous person I know you will store this information away in your vast library.? @Sniper also has a very valid point as most people rarely or have never dressed their points....it really is one of the few times when you are working on a car and these three can be combined with good results......cheap, fast, and perfect. It is very important to file the contact pads in an EVEN PATTERN or you are right back where you started from with less materials.

Not only do I file points on dizzys I file points on voltage regulators when they get build up.

[DJK]

20 hours ago, desoto1939 said:

Yes I had the dwell set for 6 cyinder and the proper red and black picjup points connected as per the instruction manual for the dwell meter.

.

So are you stating that I need to set the gap at 20? i hate to fool with things that are running well Any help or advise is greatly appreciated.

 

Rich Hartung

desoto1939@aol.com

Rich, I find my Cranbrook runs best w/ dwell at 36 degrees(checked with engine running) and timing set at 6 degrees. I tried to "eyeball" the settings after a breakdown on the road, it ran but wasn't happy. As a reference: turning the breaker pt. adjustment counter clockwise lowers the dwell reading, turning dist. clockwise, retards the timing. Been a long time since I have dealt w/ pts>!!!!

[keithb7]

Reviving this thread today as I learned something new about points.

 

I located an article in an old "Skinned Knuckles" magazine. Dated Nov 1997. All about dwell and ignition timing. I was intrigued to learn that for every 1 degree that the dwell is adjusted, ignition is also adjusted exactly 1 degree.  That's considerable. Many folks are still adjusting their points gap with a feeler gauge. That's not good enough. Sure it'll get you home.  A matchbook cover will get you home too. However you don't really know how far out of spec your dwell time is. The fibre rubbing block on the points arm wears as it rides up and down the distributor cam. It's wear affects points opening time and dwell. Additionally points contacts wear down. Gap increases. This also effects ignition timing.  Maybe later we are up front at the crank pulley with a timing light trying to twist the distributor. We run out of adjustment. Still we can't seem to get the timing mark lined up as per OEM specification. As the rubbing block wears, the points wear, the ignition timing retards accordingly. All this adds up and compounds.

 

If a car is new to us and we have little confidence in the history of it's maintenance:

Maybe we should start with new points? They are cheap. A new condenser. Get a dwell meter. Few people know what they are for any more anyway. Find a used one for good price.  Set the dwell to spec. Record dwell with the engine running . Set the valve clearances hot. Set A/F mixture screw with vacuum gauge to assist. Re-set engine idle RPM. New spark plugs and lead wires, rotor and cap sure won't hurt! Replace any rotting points wires including the wire from the coil to the distributor. Ensure the isolation block or washers are in place at the side of the distributor where the points wire enters. Then finally set the timing on the crank pulley with a timing light.

 

You should be left with a well tuned car! (assuming, vacuum and centrifugal advance, carb settings are all working well). As you drive it, the points ramp block wears down. The contact points wear...Little by little we lose engine performance as ignition timing retards. Maybe not enough to notice it until we see that we are burning more fuel. Maybe travelling that hill home slower than we used to. 

 

Keep on top those wearing variables. Get a dwell meter! Get to the root of the problem.  If I grasp all this correctly it seems to me, just setting the timing with a timing light and twisting the distributor is not the right approach. Start with points, then the dwell time...Then after, take out the timing light.

 

 

Edited November 19, 2020 by keithb7

[Sniper]

When you are setting the gap you are setting it for maximum opening with the rubbing block on the high point of the cam.  That will compensate for any wear in the system. well up to the point that it is no longer long enough to open the points enough to get your required gap.

 

You do want clean, smooth and parallel  contacts for maximum electrical conductivity.  When I run a points file across my points to dress them I do it with the rubbing block on the low side of the cam and allow the points to close on the file as I lightly drag it across the points.  That's usually reserved for pretty nasty points though.  Pulling a dollar bill across the closed points will generally clean them up fine. 

 

But yes, a dwell meter is your sanity check and you always check the timing afterwards. 

[Young Ed]

Must say you guys inspired me to make a purchase when this came up on facebook marketplace. Guy said it worked last time he used it but that was many years ago. 

 

Then I got it home and realized it had something broken sounding rattling around inside. Bummed out I put it aside for a week. Today I took it apart and the rattle was easily fixed. 

 

It was just a nut that had come off inside!

[keithb7]

Cool! Welcome to the dwell meter club. Have you set it up to check your dwell yet?  How about a little report on your findings? Thx. 

[Los_Control]

On 11/19/2020 at 4:27 PM, Sniper said:

Pulling a dollar bill across the closed points will generally clean them up fine. 

 

I am married, Not often I have a dollar bill. 

I like dragging a white business card through them, seems I always have one of them. Is amazing how much grease and dirt you pull from the points after cleaning them. And the white card or paper it shows up.

When you do it one time and see the dirt you pull off them, you will always remember to do it every time.

 

When I was a youngster and had a no start issue after doing a tuneup including new points, a oldster showed me the trick and my truck started right up.

[Young Ed]

1 hour ago, keithb7 said:

Cool! Welcome to the dwell meter club. Have you set it up to check your dwell yet?  How about a little report on your findings? Thx. 

Most of my stuff is in winter storage so likely it won't get a trial run until April

[harmony]

I hope it's not too late to post a question on this thread.  I've just come across a Penske 21014 Dwell/Tach meter for sale on line.  It looks to be 60's or 70's vintage. However the guy doesn't have the manual for it. It doesn't indicate if it will work on 6 volt or if it will work on positive ground cars.  It has two leads with clips.  Would you connect the red lead to the engine ground bolt and the black lead to the coil?  If so, which post on the coil would you clip it to?  I've read on a thread from about 12 years ago that guys were using a 12 volt battery for the dwell meter and then running a wire from the negative post to engine ground.  Also I should look around for a timing light.  Will I run into the same issues with it?

[Sharps40]

Try not to over think or over engineer points swap and adjustment.

 

Simple.

 

Assuming timing is correct, proceed as follows...

 

Install new points and condenser, cap, rotor, etc.

 

Make initial adjustment of gap to about 20 thousands, match book cover is fine.

 

Start and warm engine.

 

Install dwell meter and check reading.

 

Set the dwell for the smallest number of degrees called out in your manual for the dizzy.   (This will be largest point gap and allow the points last the longest.)

 

E.G. doing it this way, you essentially ignore the very difficult to measure points gap and the easy reading dwell meter assures you are in the correct range for the longest time period. (e.g. the life of the rubbing block)

 

 

 

 

[busycoupe]

i have a sears dwell meterfrom the 70's with twoleads as you describe. The instructions say that it is for 12 volts neg. ground, but I connect as you describe and it works fine on my 6volt positive ground car wit no extra battery needed.

[harmony]

3 hours ago, busycoupe said:

i have a sears dwell meterfrom the 70's with twoleads as you describe. The instructions say that it is for 12 volts neg. ground, but I connect as you describe and it works fine on my 6volt positive ground car wit no extra battery needed.

Good to hear, it may be the same meter since Penske is made by Sears Roebuck.  So I'm assuming your meter has one red protective cover and one black cover.  When you use it on a 6 volt positive ground.  How do you connect the leads?

[Plymouthy Adams]

regardless of chassis ground the device will need be connected + and - respectively....

[kencombs]

I've had a few of the older meters.  The dwell seems to work on either voltage or grounding polarity.  The versions that include a tach are sometimes not so flexible.

[Sniper]

4 minutes ago, kencombs said:

I've had a few of the older meters.  The dwell seems to work on either voltage or grounding polarity.  The versions that include a tach are sometimes not so flexible.

 

I would suggest connecting both ways and see if it reads the same regardless.  If not I'd follow the polarity as Plymouthy said.

[busycoupe]

My meter has agreen lead (positive) and a black lead (negative)  I connect negative to the distributer side of the coil, and positve to a good ground; usually the positive battery post. Just opposite of what is shown in the diagram.

Edited December 19, 2020 by busycoupe

[Los_Control]

Cough .... I think dwell is over rated.

If important they would have made it adjustable like a cheby distributor?

 

Just yanking your chain. With the chevrolet distributor you could open the window and adjust the dwell.

No other car manufacturer offered that .... WTF? Was dwell not important to them?

 

Just a pita to remove the dizzy cap and adjust the points from 12k to 16k to 20k & back to 13 to adjust the dwell.

Now that you got it set just where you want it ... next month with points wear it will be different.

I know I know ... put yer trailer queen back on the trailer.

I will pull out my match book to set the points and skip the dwell 

[harmony]

38 minutes ago, busycoupe said:

My meter has agreen lead (positive) and a black lead (negative)  I connect negative to the distributer side of the coil, and positve to a good ground; usually the positive battery post. Just opposite of what is shown in the diagram.

Thanks busycoupe.

[kencombs]

1 hour ago, Los_Control said:

Cough .... I think dwell is over rated.

If important they would have made it adjustable like a cheby distributor?

 

Just yanking your chain. With the chevrolet distributor you could open the window and adjust the dwell.

No other car manufacturer offered that .... WTF? Was dwell not important to them?

 

Just a pita to remove the dizzy cap and adjust the points from 12k to 16k to 20k & back to 13 to adjust the dwell.

Now that you got it set just where you want it ... next month with points wear it will be different.

I know I know ... put yer trailer queen back on the trailer.

I will pull out my match book to set the points and skip the dwell 

On that old GM stuff you can get real close without a meter or gap guage.  Just set it my eyeball to get it to run.  Then start it, open the 'window' insert 1/8" hex, I have a long flex model specifically for that, turn clockwise until it stumbles, turn countercw while counting 1/4turns.  When it stumble again, turn cw 1/2 the amount from the last step.  done.